using Kirchoff's voltage law

[pme720]

I want to make sure I am properly understanding and applying Kirchoff's voltage law (the one where the voltages in the circuit add to 0V).

Suppose I have a picaxe and an n-MOSFET as depicted in the picaxe manual 3 (pin to gate, V+ to load to drain, source to ground/0V). Now suppose my MOSFET's datasheet says 20V in the Absolute Maximum Ratings (I grabbed this datasheet from digikey: http://www.vishay.com/docs/72322/72322.pdf).

Let V+ = 40V, Load = 10 x series Green LEDs (3.6Vf each) with an appropriate resistor. Is my MOSFET OK? Here's my work:

10 x 3.6V = 36V dropped in the series LEDs.
The MOSFET datasheet lists a "diode forward voltage"/Vsd of 0.8V (not too sure about this part).
I put a 3.2V / 20ma = 160Ohm (and 0.02^2 x 160 = 0.064...1/8W) resistor in series with the LEDs to drop the remaining voltage.

Thus I have 40V going in and 36 + 0.8 + 3.2 = 40V coming out. So Kirchoff seems happy. But my MOSFET is rated at 20V. So another way for me to ask that question is, when the MOSFET (or any part, I suppose) says "20V" as its limit, is that the gross voltage from my power supply or the net voltage after the load drops most of those volts? Does my 40V supply in the example kill the MOSFET? Or is the MOSFET safe (as long as the LEDs and resistor don't fail, obviously) after the 36V are dropped in the LEDs?

Assume a linear regulator to give only 5V to the picaxe.

Many thanks,

 

[BeanieBots]

Your understanding is correct but where do you get the 20v limit from on your 40v MOSFET?

The maximum voltage seen by the MOSFET is when it is OFF.
When off, it looks like an open circuit.
Although the LEDs normally have a voltage across them when on, they also have a finite resistance which means they conduct (albeit very little) when the voltage across them is very small.
Hence, they will pull the drain up to supply voltage.

Your supply is 40v and your MOSFET rating is 40v.
That's too close. It WILL fail eventually.
You should be using a MOSFET with at least a rating of 60v to be safe.

 

[inglewoodpete]

The MOSFET has a max Vgs of 20v but that has very little to do with the output side. Vgs will be limited to the 0 to +5v range, limited by the PICAXE.

 

[pme720]

Thank you both, very much. I forgot to look at what is happening when the MOSFET is off. And I was sloppy in mixing the 20V and 40V gs and ds ratings in the example.

Again my thanks